The present invention relates to an image scanner for transforming a text image or a graphic image to corresponding digital data and, more particularly, to an image scanner for reading partial images with a linear image sensor and combining them by subscanning to thereby output a complete bidimensional image.
A camera type image scanner having a reading unit including a liner image sensor and a subscanning mechanism is conventional. This type of image sensor reads a document while being spaced from the document. An image scanner DS-3000 available from Chinon Industries Inc. is one of such image scanners. Specifically, the image scanner includes a subscanning mirror rotated by the combination of a stepping motor and a decelerator, thereby sequentially varying the image to be projected onto the linear image sensor. The prerequisite with this type of image scanner is that the stepping rate of the stepping motor be increased with a decrease in required reading time. However, it is difficult to drive stepping motors in general at speeds higher than 1,000 ppm (pulses per minute). Further, vibration ascribable to the operation of the stepping motor has adverse influence on the mirror and other constituents of the image scanner, introducing irregularity in the resulting image. Moreover, gears of utmost accuracy are essential because the image is expanded or contracted unless the decelerator is free from eccentricity.
Japanese Patent Laid-Open Publication No. 1-265663, for example, teaches an image inputing device including a scanning section capable of controlling the rotation angle of a movable mirror. The image inputing device is capable of reducing the irregularity of the output of a photoelectric transducer to a negligible degree. Specifically, the mirror reflects light incident thereto from a subject. A focusing unit focuses the resulting reflection from the mirror onto an image pick-up surface where photoelectric transduction elements are arranged in an array. The transduction elements convert the incident light to an electric signal by main scanning. Every time main scanning completes once, the mirror is angularly moved in the subscanning direction such that the next part of the image is incident to the transduction elements. The problem with the image inputing device is that it lacks means for sensing the rotation angle of the mirror and therefore needs a stepping motor and a deceleration mechanism for controlling the movement of the mirror. This also lowers image quality in the event of high-speed reeding.
Japanese Patent Laid-Open Publication No. 8-7073 also proposes an image scanner of the type controlling the rotation angle of the subscanning mirror by sensing it. The image scanner taught in this document is feasible for high-speed drive. However, this image scanner needs an optical pickup, or sensor, consisting of a number of parts including a laser, light-sensitive element, objective lens, half-mirror, and VCM (Voice Coil Motor) for lens focusing. This makes it difficult to miniaturize the optical pick-up and therefore the entire reading unit. Further, two feedback control lines, one for lens focusing and the other for the rotation of the mirror, are required, rendering circuitry and adjustment sophisticated. Moreover, high-speed operation would cause the mechanical portions to vibrate and would thereby lower image quality. In addition, such an image scanner needs a great number of parts which would increase the cost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 60-128775, Japanese Utility Model Laid-Open Publication No. 62-151268, and Japanese Patent Laid-Open Publication No. 8-154153.